This invention relates generally to the art of resistance seam welding by the application of single phase, alternating current to welding electrodes in a manner to provide a succession of alternate welding and cooling stages synchronized with the frequency of the alternating current. Present commercial seam welding systems of the character mentioned have generally been of the type wherein the heat time or weld pulses have consisted of one or more full cycles of commercial frequency, alternating current, followed by a cool time or inter-pulse time which also was of a duration consisting of one or more full cycles of current. In such systems, the current employed must have a certain minimum requirement in terms of instantaneous value since below a certain value no weld will occur for sheet stock of given thickness. A second requirement in present-day systems is that the ratio of peak current to average current be considerably greater than one, and a third requirement is that peak values of current shall be of a suitable frequency to meet the required number of welds per inch of stock travel.
The use of heat time and cool time cycles consisting of one or more complete cycles of current, of course, raises the ratio of peak current to average current. The weld speed is, further, necessarily controlled by the frequency of weld pulses required to produce the required number of welds per inch and, in such systems, speeds cannot exceed what have come to be accepted in the trade as normal or regular speeds for various material thicknesses. In high production assembly lines, such as employed for instance in the manufacture of vehicle bodies, appliance cabinetry, and heat exchanger components, it is important for the seam welding equipment to accomplish its function as rapidly as possible so that production rates may be as fast as possible.
Some system variations have been proposed, such as that illustrated in Silverman U.S. Pat. No. 2,092,093 wherein welding current is symmetrically applied at the midpoint of the half cycle to weld thin parts such as ordinary tin cans. Such systems do not, of course, teach what I have conceived nor do I believe that they have found any wide commercial use, and particularly any use with heavier gauge metals. Other systems of which I am aware are disclosed in U.S. Pat. Nos. 2,175,920; 2,243,832; 2,429,186; 2,394,004; 3,569,659; 2,415,708; and 2,413,083.